The act targets "libel tourists" who launch cases in countries whose legal systems are considered far more claimant-friendly, such as the UK.

In the UK defendants must prove statements are true, whereas in the US claimants have to prove they are false.

Book on terrorism

Some cases have been brought in the UK because US libel laws include the so-called Sullivan defence, in which the claimant must prove malice or serious recklessness before the case proceeds to the courts.

Supporters of the Speech Act say the legislation is of huge importance to national security.

It means US courts can refuse to enforce any foreign libel case judgements.

The Speech Act... allows Americans to expose the enemies of freedom and democracy without fear of foreign intimidationRachel Ehrenfeld

The only exception would be if the free-speech protections given in foreign courts were considered to match those in the US.

The change in US federal law stems from the case of the writer and academic, Rachel Ehrenfeld.

She was sued in the High Court in London over a book on the funding of terrorism on the basis that although it was only published in the US, 23 copies were sold in the UK via the internet.

Although she refused to take part in the action, she was ordered to pay £10,000 in damages to each of the wounded parties as well as costs.

Ms Ehrenfeld thanked friends and supporters in an e-mail from the American Center for Democracy (ACD), of which she is director, after the law was signed by President Obama.

"No longer will libel tourists be able to suppress the rights of American scholars, writers and journalists to speak, write and publish freely in print and on the internet," she said.

"The Speech Act is of monumental importance to national security and the protection of free speech. [It] allows Americans to expose the enemies of freedom and democracy without fear of foreign intimidation."

The federal legislation follows laws passed in some US states, including New York - where Ms Ehrenfeld is based - California, New Hampshire and Hawaii.